Primary productivity of the Helgolandic kelp forest. Light response curves of biomass dominant brwon algae (Phaeophyceae) as basis for productivity estimates.

Inka.Bartsch [ at ]


Although it is recognized that coastal macroalgae communities are as productive as the most productive terrestrial systems, few investigations of primary production of kelp forests were performed worldwide over an entire season. In the present study, a prototype primary production model was developed and first estimations of the summer primary production between July and early September of the dominant kelp species Laminaria hyperborea (Laminariales) in the sublittoral off the island of Helgoland (North Sea) were derived. The development of the model involved two steps: (1) an estimation of the basic ex situ net primary production capacity and (2) an estimation of the realized in situ summer net oxygen production, both along the depth gradient of L. hyperborea. For step (1) the photosynthetic characteristics of L. hyperborea were derived from ex situ photosynthesis irradiance (PE) measurements following the concept of Jassby and Platt (1986) which were correlated with continuous underwater irradiance data taken over seven weeks at four depth levels. For step (2) the in situ summer net oxygen production per unit seafloor along the depth gradient was derived by multiplying the basic leaf area based net production estimated in step (1) with in situ leaf area data of L. hyperborea given in Gehling (2006) and Pehlke and Bartsch (2008). Due to the occurrence of L. hyperborea in most of the photic sublittoral zone and the high level of thallus differentiation, measurement of photosynthetic activity and Chlorophyll a (Chl a) content was performed with individuals from different depths (0.5 m, 2 m, 4 m, 6 m MLWS) and with algal blade from different regions of the blade (5, 25 and 50 cm above the stipe-blade transition zone). The photosynthetic characteristics maximal photosynthetic rate (Pmax), photosynthetic efficiency at low light intensities (alpha), light saturation point (Ek), dark respiration as well Chl a content of L. hyperborea showed no significant depth-dependent variation and thus, no acclimation to decreasing underwater irradiance with increasing depth. A mean Pmax of 3.3 μmol O2 cm-2 h-1 (17.6 μmol O2 g-1 h-1) and a mean Ek of 70 μmol photons m-2 s-1 was determined at all depths. Only the light compensation point (Ec) was significantly lower in L. hyperborea from 2 m depth (MLWS) (6 μmol photons m-2 s-1) than in individuals from 4 m (MLWS) (11 μmol m-2 s-1). While, Pmax, dark respiration and alpha increased from the basal to the distal part of the blade when normalized to unit fresh weight, normalization to unit leaf area showed no variation of photosynthetic parameters along the blade. Thus, photosynthetic activity of blade segments from the intermediate region normalized to leaf area should be representative for the photosynthetic performance of the entire blade. Dry weight based Chl a content also increases significantly with almost twice as high concentrations in the distal region (1.98 mg Chl a g-1 DW) than in the meristematic region (1.02 mg Chl a g-1 DW). Chlorophyll a content based on leaf area was, however, highest in the meristematic region (0.014 mg Chl a cm-2) with a significantly decreasing Chl a content towards the distal region of the blade (0.009 mg Chl a cm-2). Pmax, alpha and dark respiration normalized to leaf area of L. hyperborea blade segments from the intermediate region were used in the further estimations of productivity along the depth gradient. The estimated basic ex situ and realized in situ leaf area based summer net oxygen production of L. hyperborea along the depth gradient showed slightly contrasting results. The ex situ net oxygen production (step 1) was highest in individuals from 0.5 m and 2 m depth (MLWS) with maximum mean daily values of 33 μmol O2 cm-2 d-1 in late July. Productivity of L. hyperborea from 4 m depth (MLWS) was slightly but insignificantly lower (26.2 μmol O2 cm-2 d-1). Only the net oxygen production of individuals from 6 m (MLWS) was significantly lower over all seven weeks and even became negative in mid-August. This was due to a generally decline of oxygen productivity by 80 – 90 % at all depths in August caused by high turbidity after a windy phase. When estimating the realized in situ net oxygen production of L. hyperborea (step 2) per m² seafloor leaf area per m² seafloor at the four depth levels the mean daily net oxygen production was no longer highest at a depth of 0.5 m (MLWS). Instead, in late July, net oxygen production was highest at 2 m depth (MLWS) with a mean value of 1.26 mol O2 m-² seafloor d-1 followed by 1.19 mol O2 m-2 seafloor d-1 at 4 m water depth (MLWS). L. hyperborea at 0.5 m (MLWS) only produced 0.96 mol O2 m-2 seafloor d-1 and at 6 m (MLWS) the net oxygen production of L. hyperborea was only 0.36 mol O2 m-2 seafloor d-1 in late July. However, due to the considerable decrease of underwater irradiance in August, the picture changed during that time and productivity of L. hyperborea at 0.5 m depth (MLWS) exceeded oxygen production at 4 m (MLWS) in August. Oxygen production at 2 m depth (MLWS) was still highest. Obtained net oxygen production of L. hyperborea in summer demonstrate the high variability of L.hyperborea productivity in the sublittoral off Helgoland but was not able to easily explain the biomass maximum of L. hyperborea which is present between 2 and 4 m (Pehlke and Bartsch 2008), partly due to the exclusion of the effects of photoinhibition and UV radiation on the photosynthetic performance of individuals near to the water surface. The decrease of biomass below 4 m found its expression in the significantly reduced net oxygen production at 6 m which even became negative from mid-August onwards. Methodically aspects of photosynthetic irradiance measurements, continuous underwater irradiance recordings and the estimation of ex situ net primary production capacity will be discussed and an outlook on future productivity measurements will be given. Future models of primary productivity of L. hyperborea should include measurements of productivity in spring and fall, the continuous recording of annual underwater irradiance as well as analysis of carbon balance.

Item Type
Thesis (Master)
Primary Division
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Matthes, L. (2015): Primary productivity of the Helgolandic kelp forest. Light response curves of biomass dominant brwon algae (Phaeophyceae) as basis for productivity estimates. , Master thesis, Institute of Biological Sciences at the University of Rostock.

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